The invention relates to a method for the sublimation growth of at least one SiC single crystal, in which a stock of solid SiC is introduced into a storage area of a crucible, and at least one SiC seed crystal is introduced into at least one crystal area of the crucible. The crucible is brought to growth conditions during a starting phase. The SiC single crystal is grown during a growth phase, by the solid SiC of the stock being at least partially sublimed and converted into an SiC gas phase with SiC gas-phase components and the SiC gas-phase components being at least partially transported to the SiC seed crystal, where they are deposited on a growing SiC single crystal. A method of this type for the sublimation growth of an SiC single crystal is known, for example, from German Patent DE 32 30 727 C2.
In the known method, solid silicon carbide (SiC), which is situated in a storage area, is heated to a temperature of between 2000xc2x0 C. and 2500xc2x0 C. and is thereby sublimed. The SiC gas phase which forms through the sublimation contains as SiC gas-phase components, inter alia, pure silicon (Si) and carbide compounds Si2C, SiC2 and also SiC. The gas mixture of the SiC gas phase diffuses through a porous graphite wall into a reaction or crystal area in which the SiC seed crystal is situated. Silicon carbide crystallizes out of the SiC gas phase on the seed crystal at a crystallization temperature of between 1900xc2x0 C. and 2000xc2x0 C. As well as the gas mixture of the SiC gas phase, there is also an inert gas, preferably argon (Ar), in the crystal area. A growth pressure of between 1 mbar and 5 mbar that is desired in the crystal area is set by suitable introduction of the argon gas. The overall pressure in the crystal area is composed of the vapor partial pressure of the SiC gas phase and the vapor partial pressure of the argon gas.
Before the actual growth phase commences, the crucible, and therefore also the crucible inner zone, are heated to a growth temperature. During the heat-up phase, a heat-up pressure, which is significantly higher than a growth pressure that is subsequently used during the growth phase, is established in the crucible by filling with the inert gas, for example with argon. After the growth temperature has been reached, the pressure is reduced by pumping gas out until a considerably lower growth pressure is reached.
European Patent EP 0 389 533 B1 also describes a method for the sublimation growth of the SiC single crystal, in which, prior to the growth phase, a crucible is heated to a growth temperature of the order of magnitude of 2200xc2x0 C. A high heat-up pressure of approximately 530 mbar is established at the same time. After the growth temperature has been reached (it should be noted that this differs slightly in the storage area and in the crystal area, so that the temperature gradient required to transport the SiC gas-phase components is established), the pressure in the crucible is then set to a considerably lower growth pressure of approximately 13 mbar.
A further method for the sublimation growth of an SiC single crystal is known from the article Journal of Crystal Growth, Vol. 115, 1991, pages 733 to 739. In this method too, the crucible used for growth is heated up under a high argon gas pressure of approximately 1000 mbar (=atmospheric pressure). As soon as the desired growth temperature has been reached, the argon gas pressure is reduced to the growth pressure. In the sublimation method, the growth pressure may also be varied during the growth phase. The growth pressure may adopt values of between 1.33 mbar and 1000 mbar.
The article in Electronics and Communications in Japan, Part 2, Vol. 81, No. 6, 1998, pages 8-17 describes a method for the sublimation growth of an SiC single crystal in which the heating-up takes place under a high pressure of, for example, 800 mbar. The high pressure is required in order to avoid crystallization at a low temperature, i.e. at a temperature which is lower than the actual growth temperature. This is because otherwise crystallization at low temperature would lead to the formation of an undesirable polytype.
Therefore, all the known methods initially heat up the crucible to the desired growth temperature under a high heat-up pressure, before then reducing the pressure to a much lower growth pressure. However, it has been found that in these methods the crystal quality of the growing SiC single crystal may be impaired and, in particular, an undesirably high rate of defect formation may result.
It is accordingly an object of the invention to provide a method for the sublimation growth of an SiC single crystal, involving heating under growth pressure which overcomes the above-mentioned disadvantages of the prior art methods of this general type, which ensures improved seeding of the growing SiC single crystal on the SiC seed crystal and therefore allows a higher crystal quality of the growing SiC single crystal. In particular, it is intended to reliably avoid uncontrolled accumulation of SiC gas-phase components on a crystallization surface.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for the sublimation growth of at least one SiC single crystal. The method includes the steps of introducing a stock of solid SiC into a storage area of a crucible, introducing at least one SiC seed crystal into at least one crystal area of the crucible, and bringing the crucible to growth conditions during a starting phase. The crucible is initially evacuated and then the crucible is filled with an inert gas, until a growth pressure is reached in the crucible, during the starting phase. Furthermore, during the starting phase, the crucible is initially heated to an intermediate temperature and then, in a heat-up phase, starting from the intermediate temperature, the crucible is heated to a growth temperature at a heating rate of at most 20xc2x0 C./min. The SiC single crystal, during a growth phase, is grown by the stock of solid SiC being at least partially sublimed and converted into an SiC gas phase with SiC gas-phase components and the SiC gas-phase components being at least partially transported to the SiC seed crystal, where the SiC gas-phase components are deposited for growing the SiC single crystal.
The invention is based on the discovery that, in the method according to the prior art which has long been used, the SiC gas phase at the SiC seed crystal is not in thermodynamic equilibrium with the SiC seed crystal in particular during the reduction in pressure to the growth pressure. Consequently, uncontrolled accumulation of SiC gas-phase components at a crystallization surface of the SiC seed crystal may occur during the reduction in pressure. This may have an adverse effect on the seeding of the growing SiC single crystal on the SiC seed crystal that takes place at the beginning of the growth process.
In contrast, seeding of the SiC gas-phase components on a crystallization surface of the SiC seed crystal can be considerably improved by the heating-up of the crucible, of the SiC seed crystal and of the SiC stock to the growth temperature being carried out substantially already at the growth pressure which is subsequently also used during the growth phase. This eliminates the reduction in pressure, which is customary in the prior art, after the growth temperature has been reached and also the uncontrolled growth of the SiC gas-phase components on the crystallization surface of the SiC seed crystal that is caused by this drop in pressure. This is because slight and controlled seeding of the SiC gas-phase components on the crystallization surface of the SiC seed crystal takes place even during the heat-up phase, on account of slow heating-up at a heat-up rate of at most 20xc2x0 C./min, preferably at a heat-up rate of at most 10xc2x0 C./min, and a growth pressure in the crucible which has already been established during the heat-up phase. In the present context, the term growth pressure is always understood as being the overall pressure in the crucible during the growth.
The slow heat-up rate while the pressure in the crucible remains the same results in that the SiC gas phase and the SiC seed crystal are virtually always close to the thermodynamic equilibrium. Consequently, a sound base for the SiC single crystal that subsequently grows during the growth phase is created even at this early time. The controlled seeding leads to a good bond between the SiC seed crystal and the growing SiC seed crystal. The resulting bond is virtually free of imperfections, which could otherwise serve as a starting point for the formation of crystal defects. Consequently, the SiC single crystal grows with a very high crystal quality.
In contrast, in the prior art seeding during the heat-up phase is deliberately suppressed by the inert gas which is introduced into the crucible and by the high pressure which the gas is used to establish. The high argon gas pressure prevents diffusion of the SiC gas-phase components to the SiC seed crystal, so that seeding is virtually impossible. As soon as the high pressure is reduced to the considerably lower growth pressure after the growth temperature has been reached, the seeding operation commences immediately. However, since the individual SiC gas-phase components have different diffusion constants in the argon gas, they also pass from the stock to the SiC seed crystal at different rates after the high argon gas pressure has been reduced. Then, the SiC gas phase whose components are not in the concentration appropriate for thermodynamic equilibrium is formed in front of the SiC seed crystal. As a result, however, the incipient formation of crystals is considerably less controlled than under conditions that are close to the thermodynamic equilibrium. It is then impossible to have a controlled influence on the seeding.
A configuration of the method in which the SiC gas phase which forms at the SiC seed crystal, during the starting phase, and in particular during the heat-up phase, is advantageously set in such a way that it has a minimum concentration of SiC gas-phase components which is required for incipient crystal growth on the SiC seed crystal. The concentration can be set in particular by a suitable temperature profile within the crucible. For this purpose, the stock of solid SiC and the SiC seed crystal are preferably brought to different temperatures, so that a low temperature gradient with a temperature drop toward the SiC seed crystal is established.
Then, a slightly supersaturated concentration of the SiC gas-phase components is established at the SiC seed crystal. As a result, first uncontrolled evaporation in particular of silicon out of the SiC seed crystal is prevented, and second growth of crystalline material on the SiC seed crystal with a high growth rate is suppressed. The slight supersaturation only causes weak seeding on the SiC seed crystal. Therefore, this measure additionally assists the seeding which is already desired during the heat-up phase.
It is particularly expedient if a temperature gradient of at most 20xc2x0 C./cm is established between the storage area containing the stock of solid SiC and the SiC seed crystal. Since seeding on the SiC seed crystal is only to be set in motion during the heat-up phase, only a low transport rate toward the SiC seed crystal is required for the SiC gas-phase components.
This can be achieved, for example, by using a very low temperature difference between the SiC stock and the SiC seed crystal. To facilitate the process engineering and to be able to establish reproducible conditions, it is expedient if a greater temperature difference is established between the SiC stock and the SiC seed crystal and the gas flow of the SiC gas-phase components is then decelerated again by configuration measures in the crucible, which reduce the diffusion rate and/or increase the flow resistance. Both methods lead to a desired low transport rate.
If appropriate, the desired temperature gradient between the storage area and the crystal area can be established by a heater device that is split in two, can be controlled separately and is situated outside the crucible. If necessary, the heater device may also contain more than two separate partial heater devices. In this way, the temperature profile in the crucible inner zone can be controlled with accuracy. The heater device may preferably be of an inductive configuration, although it may also be configured as a resistance heater.
A further embodiment, in which the heat-up rate during the heat-up phase is varied from an initial level, in particular is reduced, is advantageous. The initial level of the heat-up rate may in this case be up to 20xc2x0 C./min. It is particularly advantageous if the heat-up rate is reduced as the temperature in the crucible inner zone approaches the growth temperature that is ultimately to be established. Toward the end of the heat-up phase, the heat-up rate may in particular amount to only at most 10xc2x0 C./min, preferably at most 1xc2x0 C./min.
In a further advantageous variant, the heat-up rate, starting from the initial value, is therefore reduced in steps as the duration of the heat-up phase increases. This slow and, in particular, controlled approach of the temperature to the growth temperature results in the SiC gas phase which is established at the SiC seed crystal being virtually in thermodynamic equilibrium with the SiC seed crystal at any time during the heat-up phase. In this case, the deviation from the thermodynamic equilibrium is just sufficient for the desired seeding process to be brought about. Particularly toward the end of the heat-up phase, the conditions are no longer disrupted by any substantial change in pressure in the crucible. This is because the growth pressure is usually established at the beginning of the heat-up phase or even beforehand, by suitable filling of the evacuated crucible with the inert gas.
However, it is also possible for the heat-up rate to be reduced continuously from the initial level. This can take place either with a constant gradient of the heat-up rate or using any other desired profile that is predetermined for the reduction in the heat-up rate. In general, it is expedient for the reduction in the heat-up rate to be greater the closer the temperature in the crucible inner zone comes to the final growth temperature.
A further embodiment of the method, in which the desired growth pressure is established by filling with at least one of the inert gases argon (Ar), helium (He) and hydrogen (H2), is preferred. In this context, hydrogen is also to be interpreted as being an inert gas. The growth pressure is preferably set to a value of between 1 and 20 mbar. This is achieved by suitable control of the quantity of gas introduced into the crucible. During the growth of xcex1-SiC, it is expedient if the growth pressure is established before the temperature in the crucible exceeds 1800xc2x0 C., in particular before it exceeds 1600xc2x0 C. Otherwise, any desired time during the starting phase is in principle suitable for establishing the growth pressure.
Up to the above-mentioned temperature of 1800xc2x0 C., the sublimation rate of the silicon carbide in the stock and at the seed crystal is virtually negligible, even in vacuum or at only a very low pressure. Therefore, up to this temperature there is no significant formation of SiC gas phase, so that as yet thermodynamic equilibrium between the SiC gas phase and the SiC seed crystal is not of any crucial importance. Since the sublimation rate in vacuum begins to rise slowly beyond a temperature of approximately 1600xc2x0 C., it may be advantageous for the growth pressure to be established even before 1600xc2x0 C. is reached.
A value of between 2100xc2x0 C. and 2300xc2x0 C. is suitable as a preferred growth temperature that is to be established during the heat-up phase. The range for the growth temperature is primarily suitable for the growth of xcex1-SiC. In contrast, xcex2-SiC is usually grown at a lower temperature, of the order of magnitude of approximately 1800xc2x0 C. Since a certain temperature profile, with in some cases different temperature values in the respective crucible areas, for example in the storage area and in the crystal area, is present in the crucible inner zone, in this context the term the growth temperature is understood as being a mean temperature across the crucible inner zone.
In a further preferred variant, the intermediate temperature to which the crucible is heated during the starting phase is set to a value of between 1000xc2x0 C. and 1400xc2x0 C.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for the sublimation growth of an SiC single crystal, involving heating under growth pressure, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.